1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile, and more particularly, to an image forming apparatus in which a toner image obtained by developing a latent image on a latent image carrier is transferred onto a recording material and a process of cleaning the latent image carrier is performed.
2. Description of the Related Art
It has been known that it is difficult to appropriately perform cleaning by a well-known blade cleaning technique in the case of using a spherical toner having a small particle diameter. The reason is as follows. In the blade cleaning technique, as the latent image carrier is rotationally driven, the blade removes the toner while rubbing the surface of the latent image carrier. For this reason, an abutting edge portion of the cleaning blade is deformed due to the frictional resistance between the cleaning blade and the latent image carrier, and a tiny space is formed between the latent image carrier and the blade. The toner having a small particle diameter can easily intrude into the space. As the shape of the toner that introduced into the space is more spherical, rolling frictional force decreases, and the toner easily rolls into in the space between the latent image carrier and the cleaning blade and is likely to escape from the cleaning blade. Thus, in the case of using the spherical toner having a small particle diameter, a large amount of the toner escapes from the cleaning blade. The toner that has escaped from the cleaning blade is not removed from the latent image carrier and continuously stays on the surface of the latent image carrier, causing a so-called filming phenomenon that the toner is fixed on the surface of the latent image carrier in a film state due to the action of a release agent or a fluidizer contained in the toner. The filming phenomenon produces a void in a solid portion of an image, that is, an abnormal image is produced.
From the past, there is known an image forming apparatus in which the surface of the latent image carrier is coated with a lubricant made of a metal salt of a fatty acid to improve a cleaning property of the latent image carrier and thereby to prevent filming when using the toner having a small particle diameter (see Japanese Patent Application Laid-open No. 2006-235563). According to the image forming apparatus, by forming a thin film on the surface of the latent image carrier with a lubricant, a frictional coefficient between the toner and the surface of the latent image carrier is reduced, so that the cleaning property of the cleaning blade improves.
However, even the image forming apparatus in which the surface of the latent image carrier is coated with the lubricant still has a problem in that an abnormal image is produced due to the filming.
FIG. 9 is an explanation view illustrating an example of an abnormal image that is generated when a release agent or a fluidizer contained in the toner adheres to the surface of the latent image carrier and thus filming occurs.
The abnormal image is an image in which a killifish-shaped void is formed in a solid portion of an image. The void appears with a frequency corresponding to the circumferential length of the latent image carrier. The inventors of the present application have investigated it and found that the abnormal image is likely to be formed when an image is formed under the following two conditions.
First, it generates under the condition in which images with a high image area ratio are continuously produced.
When forming an image with a high image area ratio, a large amount of residual transfer toner is generated and a large amount of toner escapes from the cleaning blade. As a result, a large amount of toner intrudes into a lubricant coating brush. If such state is continuous, the toner may become solidified in the lubricant coating brush. In the case of using a process in which a powder lubricant scraped from a solid lubricant is coated by the lubricant coating brush, a large amount of toner that has intruded into and thus exists in the lubricant coating brush is also mixed with the solid lubricant, so that the toner may become solidified by the solid lubricant. If this happens once, the lubricant cannot be uniformly and stably coated. Accordingly, portions having the high frictional coefficient are locally formed on the surface of the latent image carrier. A substance that causes filming easily adheres to the portions. As a result, the filming occurs and an abnormal image is produced as illustrated in FIG. 9.
The second condition is the case in which images with a low image area ratio are continuously produced.
When forming an image with a low image area ratio, the amount of residual transfer toner is small. Accordingly, a small amount of toner escapes from the cleaning blade. The small amount of toner intrudes into the lubricant coating brush. If the small amount of toner has intruded into the lubricant coating brush, in the case of coating the powder lubricant scraped from the solid lubricant with the use of the lubricant coating brush, it is known that the amount of the lubricant scraped by the lubricant coating brush is small. Generally, since an amount of the lubricant supplied from the lubricant coating brush is not uniform, if the amount of the lubricant scraped by the lubricant coating brush decreases and the supply amount of lubricant from the lubricant coating brush decreases, there may be a portion with an insufficient amount lubricant, on the surface of the latent image carrier. The portion increases in frictional coefficient. Thus, in the case of continuously forming the images with a low image area ratio, a portion having the high frictional coefficient is formed on the surface of the latent image carrier, and a substance that causes filming easily adheres to the portion. Thus, filming occurs, so that the abnormal image is produced as illustrated in FIG. 9.
The above problem may not be limitedly involved in the blade cleaning technique of using a cleaning blade as a cleaning member but be involved in all cleaning techniques of performing cleaning based on rubbing the latent image carrier using a cleaning brush or other cleaning members.
The present invention is made in view of the above mentioned problems, and it is an object of the present invention to provide an image forming apparatus capable of preventing production of an abnormal image when forming an image even under the condition in which an abnormal image with a killifish-shaped void in a solid image portion is likely to be formed.
According to the present invention, during the continuous image forming job, the job is interrupted. During the interruption period, the latent image carrier idly rotates, and the cleaning process of rubbing with the cleaning member is performed. The idle rotation of the latent image carrier is referred to as a rotation of the latent image carrier in a state in which the toner is not supplied to the latent image carrier from the developing unit. Thus, during the idle rotation of the latent image carrier, nearly no toner is present on the latent image carrier. In this state, frictional force of the cleaning member on the latent image carrier is improved compared with the state in which the toner is supplied to the latent image carrier. As a result, the substance that is the cause of filming and is not able to be removed from the latent image carrier during image formation can be removed by rubbing of the cleaning member. Therefore, according to the present invention, the material causing the filming that has grown on the latent image carrier until interruption due to continuous image formation during the continuous image forming job can be removed, and the surface state related to filming on the latent image carrier can be reset.
Here, if the cleaning process is performed, frequently interrupting the continuous image forming to prevent production of an abnormal image in which a killifish-shaped void is formed in a solid portion of an image (hereinafter, referred to as simply “abnormal image with a void”) during the continuous image forming job, the productivity of the image deteriorates. Thus, the interruption frequency during the continuous image forming job is preferably as low as possible.
As a result of keen study described in detail later, the inventors of the present invention has found out that the time at which the abnormal image with a void is generated during the continuous image forming job can be specified with high accuracy based on the image area ratio of the toner image that is formed on the latent image carrier by the continuous image forming job and a given number of formed images where a predetermined number of abnormal images is generated when continuous image formation is performed with the image area ratio.
Therefore, according to the present invention, directly before the time at which the abnormal image with a void is generated through continuous image forming job, the job is interrupted, and the cleaning process is performed, thereby preventing the generation of the abnormal image with a void. As a result, the interruption frequency during the continuous image forming job can be reduced to a level as low as possible, thereby preventing the productivity decline.